Lithographic apparatus and method for calibrating the same

ABSTRACT

Lithographic apparatus includes a substrate table and a motion control system for controlling a movement of the substrate table. The motion control system includes at least 3 position detectors constructed for detecting a position of the substrate table. For measuring a position and orientation of the substrate table, each position detector comprises an optical encoder of a single dimensional or multi dimensional type, the optical encoders being arranged for providing together at least 6 position values, at least one position value being provided for each of the 3 dimensions. 3 or more of the at least 3 optical encoders being connected to the substrate table at different locations in the 3 dimensional coordinate system. The motion control system is arranged to calculate the position of the substrate table in the 3 dimensional coordinate system from a subset of at least 3 of the 6 position values and to calculate an orientation of the substrate table with respect to the coordinate system from another subset of at least 3 of the 6 position values. Further, a method for calibrating the position detectors is described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and method forcalibrating the same.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a lithographic patterning device, which isalternatively referred to as a “mask” or “reticle,” may be used togenerate a circuit pattern corresponding to an individual layer of theIC, and this pattern can be imaged onto a target portion (e.g.,comprising part of, one or several dies) on a substrate (e.g., a siliconwafer) that has a layer of radiation-sensitive material (i.e., resist).

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one go, whilein so-called scanners, each target portion is irradiated by scanning thepattern through the projection beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart a projection beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the projection beam may not exactly correspond to thedesired pattern in the target portion of the substrate. Generally, thepattern imparted to the projection beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit (IC).

The patterning device may be transmissive or reflective. Examples ofpatterning means include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

Lithographic apparatuses generally employ a motion control system. Themotion control system comprises a position detector for detecting aposition of the substrate table in at least a plane, i.e. in at leasttwo dimensions and a controller constructed for driving the actuator independency on an output signal provided by the position detector. Themotion control system thus ensures that the substrate table is in acorrect position (within a certain tolerance range) as a position of thesubstrate table is detected by the position detector, and a differencebetween the detected position and the desired position is reduced by anappropriate action of the controller. The position detector andcontroller thus form part of a feed forward and/or feed back controlsystem.

In a current lithographic apparatus, a desired accuracy for thesubstrate table (also sometimes called a wafer table or wafer stage) isin an order of magnitude of nanometers. Hence, it is required accordingto the state of the art that the position detector achieves such highaccuracy. Furthermore, requirements on the position detector are alsohigh in that the range within which the position detector is required tooperate encompasses a range of movement of around 0.5 m, as thesubstrate table in a lithographic apparatus according to the state ofthe art is able to make movements in two dimensions, i.e. in a planecovering around 0.5 m×0.5 m. To achieve these requirements, according tothe state of the art, the position detector comprises one or moreinterferometers, preferably an interferometer for a first dimension andan interferometer for a second dimension perpendicular to the firstdimension. A disadvantage of the interferometers however is that it isan expensive position detector.

A further type of position detector, well-known in the general state ofthe art, is an optical encoder. The encoder consists of a light source,a grating and a detector. By moving the grating with respect to thelight source and the detector, changes occur in the light pattern asreceived by the detector due to e.g. reflection or transmission changes.The grating is thus comprised in an optical path from the light sourceto the detector and by movement of the grating, the pattern as receivedby the detector changes. From these changes, displacement of the gratingwith respect to the light source and detector can be calculated. Fromthese displacements and knowing a starting position, a position can becalculated. As will be known to the skilled person, the above describesan incremental encoder, the skilled person will be familiar with thefact that also absolute encoders exist.

A specific type of optical encoder has been described in co-pending U.S.patent Pub. application No. 2002/0041380, which is incorporated hereinby reference. This optical encoder comprises a diffraction type encodercomprising a light beam generator constructed for generating a lightbeam, a first grating, a second grating, the second grating beingmovable with respect to the first grating, and a detector arranged fordetecting a diffracted beam of the light beam as diffracted on the firstand the second grating, one of the gratings being mechanically connectedto the substrate table, the other one of the gratings being mechanicallyconnected to a reference base of the lithographic apparatus, a movementof the substrate table causing a movement of the first grating withrespect to the second grating and in operation causing a change in thediffracted beam.

The known lithographic apparatus comprises a motion control system forcontrolling a movement of the substrate table. The substrate table ismovable in at least two directions under control of the motion controlsystem. The movement of the substrate table is to be understood as amovement of the substrate table with respect to the projection system,i.e. a movement of the substrate table results in a movement of thepatterned radiation beam with respect to the substrate.

SUMMARY OF THE INVENTION

The principles of the present invention, as embodied and broadlydescribed herein, provide a calibration apparatus that calibrates aradiation sensor in a lithographic apparatus having an illuminationsystem. In one embodiment, the apparatus comprises a substrate holderconfigured to hold a substrate; an illuminator configured to condition abeam of radiation; a support structure configured to support apatterning device that imparts a desired pattern to the beam ofradiation; a projection system that projects the patterned beam onto atarget portion of the substrate; and a motion control system configuredto control a movement of the substrate table, the motion control systemcomprising a plurality of position detectors that detect a position ofthe substrate table. At least three of the plurality of positiondetectors include a single or multi-dimensional optical encoder toprovide at least six position values, the optical encoders being coupledto the substrate table at different locations within a three dimensionalcoordinate system and at least one position value is provided for eachdimension of the three dimensional coordinate system. The motion controlsystem is configured to calculate the position of the substrate tablewithin the three dimensional coordinate system from a subset of at leastthree of the six position values and to calculate an orientation of thesubstrate table with respect to the three dimensional coordinate systemfrom another subset of the at least three of the six position values.

According to a further aspect of the invention, there is provided amethod for calibrating the position detector in the lithographicapparatus according to any of the preceding claims, the methodcomprising creating a first pattern on a substrate comprising a firstmatrix of reference marks; creating a second pattern on the substratecomprising a second matrix of reference marks; comparing the secondmatrix of reference marks with the first matrix of reference marks;determining respective position deviations between reference marks ofthe first matrix and corresponding reference marks of the second matrix;storing the position deviations in the calibration matrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 highly schematically depicts an exemplary embodiment of anoptical encoder of the lithographic apparatus according to theinvention;

FIGS. 3 a and 3 b each depict a configuration of a substrate table and aplurality of position detectors according to embodiments of theinvention; and

FIG. 4 depicts an embodiment of a method for calibrating a positiondetector of a lithographic apparatus.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a lithographic apparatus 1 according to anembodiment of the invention. The apparatus 1 comprises:

an illumination system (illuminator) IL: for providing a projection beamPB of radiation (e.g., UV or EUV radiation).

a first support structure (e.g., a mask table/holder) MT: for supportingpatterning device (e.g., a mask) MA and coupled to first positioningmechanism PM for accurately positioning the patterning device withrespect to item PL;

a substrate table (e.g., a wafer table/holder) WT: for holding asubstrate (e.g., a resist-coated wafer) W and coupled to secondpositioning mechanism PW for accurately positioning the substrate withrespect to item PL; and

a projection system (e.g., a reflective projection lens) PL: for imaginga pattern imparted to the projection beam PB by patterning device MAonto a target portion C (e.g., comprising one or more dies) of thesubstrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support structurecan use mechanical, vacuum, electrostatic or other clamping techniquesto hold the patterning device. The support structure may be a frame or atable, for example, which may be fixed or movable as required. Thesupport structure may ensure that the patterning device is at a desiredposition, for example with respect to the projection system. Any use ofthe terms “reticle” or “mask” herein may be considered synonymous withthe more general term “patterning device.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on-the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position detector IF (according to the state ofthe art e.g. an interferometric device, linear encoder or capacitivesensor, according to the invention an optical encoder of the type to bedescribed below), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B.

Similarly, the first positioner PM and another position detector (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe mask MA with respect to the path of the radiation beam B, e.g. aftermechanical retrieval from a mask library, or during a scan. Thepositioner, position detector as well as a controller are comprised on amotion control system. The controller is constructed or programmed fordriving the positioner based on a signal representing a desired positionand an output signal of the position detector representing an actualposition. The motion control system can form e.g. a feed-forward or afeed-back control loop. In general, movement of the mask table MT may berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM.

Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. In the case of a stepper (as opposed to a scanner)the mask table MT may be connected to a short-stroke actuator only, ormay be fixed. Mask MA and substrate W may be aligned using maskalignment marks M1, M2 and substrate alignment marks P1, P2. Althoughthe substrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks). Similarly, in situations inwhich more than one die is provided on the mask MA, the mask alignmentmarks may be located between the dies.

The depicted apparatus can be used in the following preferred modes:

step mode: the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C in one go (i.e., asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

scan mode: the mask table MT and the substrate table WT are scannedsynchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e., a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

other mode: the mask table MT is kept essentially stationary holding aprogrammable patterning device, and the substrate table WT is moved orscanned while a pattern imparted to the projection beam is projectedonto a target portion C. In this mode, generally a pulsed radiationsource is employed and the programmable patterning device is updated asrequired after each movement of the substrate table WT or in betweensuccessive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmablepatterning device, such as a programmable mirror array of a type asreferred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

In this specification, the terms direction, dimension, and axis, asused, are intended to relate to one and the same coordinate system.Moreover, the term position value is to be understood as meaning anoutput signal of a position detector, the output signal beingrepresentative of a position.

FIG. 2 shows a schematic view of an embodiment of a position detector ofthe lithographic apparatus according to the invention. FIG. 2 depicts a1-dimensional diagram of such a detector. The position detector IFcomprises a first grating g1 and a second grating g2. The first gratingg1 and the second grating g2 are positioned parallel to each other. Theposition detector IF further comprises a light source S which generatesa light beam LB. The light beam generates, when incident on the firstgrating g1, a diffraction pattern, schematically indicated by DIF1, thusgenerating a diffracted light beam DLB. The diffracted light beam againcreates a diffraction pattern on the second grating g2, a part of theincident radiation of the light beam travelling a same path (asindicated by DLB, LB) back to the source S.

To detect the diffracted, received radiation, the source is combinedwith a light sensor LS detecting the radiation which has been diffractedtwice, i.e. on the first grating g1 and the second grating g2, and isreceived back by the light sensor. When a movement of the substratetable WT takes place, the first grating g1 moves with respect to thesecond grating g2. This movement in the example depicted in FIG. 2 takesplace in the direction indicated by x.

Upon a movement in the direction indicated by x, the diffractionpatterns of the light beam LB on the first grating and the secondgrating will change thus resulting in a change in the light receivedback by the light sensor LS. When the first grating and second gratingare moved with respect to each other by a distance equal to a pitch p ofthe grating, a same diffraction pattern will occur, thus a same amountof radiation will be received by the light sensor. Thus, periodicchanges in the diffraction pattern upon movement of the gratings withrespect to each other will result in a periodic change of the radiationreceived back by the light sensor LS. In FIG. 2, the gratings areschematically indicated by lines, which will provide an adequatesolution for a single dimensional position detector.

The position detector disclosed above with respect to FIG. 2, may bedescribed as a 1-dimensional encoder. The embodiments described belowwill, however, make use of 1-dimensional as well as 2-dimensionalencoders. A 2-dimensional encoder or position detector can be created byusing two separate position detectors as described with reference toFIG. 2, thus each position detector comprising a set of a first and asecond grating. However, in another embodiment a single, 2-dimensionalposition detector may be employed. In such a 2-dimensional positiondetector, a common set of gratings is used for the two positiondetectors, each detector for detecting a movement of the grating in adifferent one of the dimensions.

Effectively, 2 or more sets of encoders, each comprising a light sourceand a light sensor make use of a common set of gratings. To perform ameasurement in more than one dimension, the gratings do not onlycomprise lines in a single direction, but instead the gratings comprisea grid comprising e.g. lines in 2 directions, a second directionpreferably being perpendicular to a first direction.

Alternatively, the grating may also be configured in a so-called“checkerboard” pattern. In such a pattern, to detect displacements inthe x and y directions, a 2-dimensional position detector comprises twolight sources S and two light sensors LS, in which one combination of alight source and a light sensor detects displacement in the x directionand one combination of a light source and a light sensor detectsdisplacement in the y direction.

The position detector IF as described with reference to FIG. 2 comprisesa diffraction type encoder, the source S may also be referred to as alight beam generator and the light sensor LS may also be referred to asa detector.

It will be appreciated by the skilled person that instead of, or inaddition to, the diffraction type encoder described above, also othertypes of encoders can be used. Furthermore, in the example described inFIG. 2, the position detector IF comprises an incremental encoder,although an absolute encoder may be used.

As will become more clear below, an advantage of the diffraction type ofencoder described above, is that a distance between the gratings, i.e.the distance along the direction indicated by z in FIG. 2, can vary overa large range in this particular type of encoder, hence making itparticularly suitable for use with a substrate table, as the substratetable is commonly able to move in a plane of around 0.5 times 0.5meters. The gratings may thus be positioned on a short distance relativeto each other, however they may also be positioned tens of centimeters,or even 0.5 m apart.

As the displacement range of the substrate or wafer table WT is verylarge, the position detector of the diffraction type may be used invarious types of 1-dimensional and 2-dimensional encoders as will beexplained with reference to FIGS. 3 a , 3 b below. Assuming that thewafer table WT is moveable in a plane defined by the co-ordinates x andy, then the diffraction type encoder may be applied for detectingmovements of the substrate table in any direction. The diffraction typeencoder may be applied in a configuration where the direction x in FIG.2 corresponds to the x dimension and the direction indicated by zcorresponds to, for example, a direction perpendicular to the x-y planeof movement of the wafer table WT.

In another embodiment, it is possible that the diffraction type encoderis applied in a configuration where the direction z as indicated in FIG.2 corresponds to an x axis or y axis. In that case, the displacement inthe direction indicated by z may be as large as the range of movement ofthe wafer table WT. In that situation, the feature of the diffractiontype encoder that it is very intolerant to a change in the distancebetween the gratings g1 and g2, becomes particularly advantageous.

To further increase accuracy of the position detector, the gratings ofthe position detector may comprise a plate made of a low thermalexpansion material, preferably comprising a glass or a ceramic. Further,the gratings may comprise a channel, the channel being connected to afluid circulation system comprising a thermal stabilization unit forstabilizing a temperature of the grating by acting on a temperature ofthe fluid in the fluid circulation system. In this manner, thetemperature of the grating can be stabilized, i.e. the grating beingheated or cooled by the fluid in the fluid circulation system.

Advantageous configurations of a lithographic apparatus comprising aplurality of position detectors according to the invention will now bedescribed with reference to FIGS. 3 a and 3 b. The embodiments asdescribed with reference to FIGS. 3 a and 3 b may make use of thediffraction type encoder as described above, although other types ofencoders may be used.

FIG. 3 a depicts a top view of the substrate or wafer table WT. Thewafer table WT in this embodiment includes five (5) position detectorsp1-p5, preferably of the encoder types described above. The wafer tableWT comprises a first side, indicated in FIG. 3 a by side 1 which issubstantially parallel to an x axis of a co-ordinate system. The wafertable WT further comprises a second side, in FIG. 3 a indicated by side2 which is substantially parallel to an y axis of the co-ordinatesystem. The wafer table WT further comprises a third side, in FIG. 3 aindicated by side 3 which is opposite to the first side and hence againsubstantially parallel to the x axis.

A first position detector p1 and a second position detector p2 arecoupled to both sides of a center of the first side. The first positiondetector p1 comprises a 2-dimensional encoder and is arranged formeasuring a position of the wafer table WT in the x dimension and a zdimension (thus providing a position value for the x and z dimensions),where the z dimension is perpendicular to the x and the y dimension. Thesecond position detector p2 is configured as a 1-dimensional encoder andis arranged for measuring a position in the z dimension.

A motion control system (not shown in FIGS. 3 a, 3 b) is programmed fordetermining from an output signal of the first position detector p1, aposition of the wafer table WT in the x dimension and from outputsignals of the first and the second position detectors p1, p2, arotation of the wafer table WT around the y axis.

Similarly, a third position detector p3 and a fourth position detectorp4 are connected to both sides of a center of the second side, in FIG. 3indicated by Side 2. The third encoder p3 is configured as a2-dimensional encoder in the y dimension and the z dimension while thefourth encoder p4 is a 1-dimensional encoder in the z dimension.

The motion control system determines from an output signal of the thirdposition detector p3 a position in the y dimension and from combinedoutput signals of the third and fourth position detectors p3, p4 arotation of the wafer table WT with respect to the x axis.

The fifth position detector p5 is mechanically connected to the thirdside, in FIG. 3 indicated by Side 3, and is arranged to measure aposition of the wafer table WT in the x dimension. The motion controlsystem is programmed for deriving from output signals of the first andthe fifth position detectors p1, p5 a rotation of the wafer table WTwith respect to the z axis. Alternatively to the configuration asdescribed with reference to FIG. 3 a, variations thereof are possible,placing p3 and p4 at a side of the wafer table WT opposite to the secondside, changing places of p3 and p4, etc.

The position detectors p1-p5 thus provide a total of 7 output signals,i.e. position values: p1 and p3 each provide 2 position values (eachbeing 2-dimensional encoders) while p2, p4 and p5 each provide oneposition value (each being 1-dimensional encoders) and the motioncontrol system calculates a position and orientation with these positionvalue.

The first and second position detectors, and similarly the third andfourth position detectors can make use of a common gratings. Thus, oneset of two gratings is shared by the first and second positiondetectors, and one set of two gratings is shared by the third and fourthposition detectors.

In a practical implementation, the range wafer table WT movement in thex and y dimensions (i.e., the plane along the surface of the substrate),will be larger or substantially larger than a range of movement in thedirection perpendicular thereto (i.e., the z-dimension). Thus, thegratings as shared by two position detectors (e.g. p1 and p2, or p3 andp4) will have a large dimension along the x-axis (p1, p2) respectivelythe y-axis (p3, p4), but only require a smaller dimension along thez-axis because of the smaller range of movement along the z-axis.

Furthermore, in a practical embodiment, the encoders will be calibrated( e.g. making use of a calibration matrix) in the x and y directions.This is because the smaller range of movement in the z direction, theinherent calibration accuracy of the encoders will be sufficient, thusmostly obviating a calibration to increase accuracy.

For increased accuracy, the pairs of position detectors p1, p2 and p3,p4 will be spaced apart as much as possible. On the other hand,increasing the space between these position detectors will require anincrease in the length in x (p1, p2) or y (p3, p4) direction of thegratings associated with that pair of position detectors, to avoid thata usable range of movement in that pair of position detectors (and thusof the wafer table WT) would be decreased. Thus, in practice acompromise between these requirements may be required.

Further, it is possible that position detector p2 may be omitted,resulting in decreased accuracy when the wafer table WT is near itsrightmost position. This is because the rightmost position an accuracyof p3 and p4 is decreased as a distance between the gratings of p3 aswell as p4 is increased.

FIG. 3 b depicts a top view of a wafer table WT and a plurality ofoptical position detectors in accordance with another embodiment of theinvention. In FIG. 3 b, the movable wafer table WT is shown in itscenter position (i.e. in a center with respect to a range of movement inany of the first, second and third dimension). In this embodiment, afirst position detector p10 is connected to a first side Side1 of thewafer table WT. The first side Side 1 is substantially parallel to thefirst dimension.

The first position detector p10 comprises a two dimensional encoderarranged for providing a position value in the first (x) and the third(z) dimension. A grating g10 of the first position detector p10 extendsalong a length of the first side Side 1. The grating g10 is mechanicallycoupled to the wafer table WT so as to follow a movement thereof. Thefirst position detector p10, or at least a light source thereof (bothnot shown in detail in FIG. 3 b), is preferably positioned to direct alight beam towards a center the grid g10 when the wafer table WT is in acenter position thereof. Hence, a range of movement in the directionalong the x-axis can be made large. That is, the movement can be madesubstantially equal to a length of the first side Side1 while the lightsource and optical detector are in operative contact with the gratingg10 over the full range of movement.

The position detector p10 preferably comprises a diffraction typeencoder as described above and may thus comprise a second grating (notshown) which does not follow a movement of the wafer table WT, i.e. isstationary with respect thereto.

FIG. 3 b further shows a second position detector p1 at a second sideSide 2, which is substantially perpendicular to the first side. Thesecond position detector p11 is configured as a two dimensional encodercomprising a grid g11 and provides a position signal in the second (y)and third (z) dimensions.

Also, FIG. 3 b shows a third position detector p12 at a third side Side3. The third position detector p12 is configured as a two dimensionalencoder comprising a grid g12 and provides a position signal in thefirst (x) and third (z) dimension. The same as outlined above withreference to the first encoder g10 also applies to the second and thethird encoder.

The motion control system (not shown in detail in FIG. 3 b) is arrangedto calculate from position values provided by the first second and thirdposition detectors p10, p11, p12 a position of the wafer table WT in allthree dimensions of the 3 dimensional coordinate system as well as anorientation, i.e. a rotational position of the substrate table WT withrespect to all three dimensions.

Preferably, a further, fourth position detector p13 is provided at afourth side Side 2 which is substantially parallel to the second side.The fourth position detector is configured as a two dimensional encodercomprising a grid g13 and provides a position signal in the third (z)dimension. The fourth encoder is particularly useful when the wafertable WT is near its rightmost position (as seen in the plane ofdrawing), because in that position the distance between a light sourceand detector of the second position detector p11 on the one hand and thegrid g11 thereof on the other hand is at or near its maximum, therebyreducing the accuracy of the second position detector p11.

In such a position, the distance between the light source and detectorof the fourth position detector p13 on the one hand and the grid g13thereof on the other hand is at or near its minimum. Hence, the accuracyof the fourth position detector p13 is not deteriorated at or near therightmost position of the wafer table WT. A further effect is that themore the wafer table WT has moved towards its rightmost position, themore the position signals of p1, p2 and p3 will provide positions in oneline, instead of providing positions at 3 points forming a triangle (asan effective point at which p11 measures a position moves to the rightin the plane of the drawing), thereby reducing the ability for themotion control system to determine multidimensional information from thez-position signals of p10, p11 and p12. Thus, the motion control systemwill preferably give comparatively more weight to a position signal ofthe fourth encoder p13 near a rightmost position of the substrate tableand less weight to a z position signal as provided by p11.

The configurations as depicted in FIG. 3 a and 3 b enable, with aminimum amount of position detectors, to measure a position of the wafertable WT in all degrees of freedom, i.e. in the dimensions x, y and z aswell as orientations around the x, y and z axis, i.e. rotationalpositions with respect to the x, y and z axis. Further, as explainedabove with reference to FIG. 2, the encoders as used in the lithographicapparatus according to the invention, not only allow a large range ofdisplacement in a direction of measurement (in FIG. 2 indicated by x),but also allow a large range of displacement in a directionperpendicular thereto (in FIG. 2 indicated by z).

The wafer table WT, as depicted in FIGS. 3 a and 3 b, has, in practicalembodiments, a range of movement of around 0.5 m in the x direction and0.5 m in the y direction. For at least the position detectors p1-p4,this range of movement results in a possible range of a distance of thegratings of that particular detector with respect to each other (i.e. avariation of the distance z according to FIG. 2) of around 0.5 m. As thediffraction type encoder described above is able to operate accuratelyover such a large range of distance between the gratings, theconfiguration as described with reference to FIGS. 3 a and 3 b mayoperate accurately over the stated range of movement, thus benefitingfrom the specific advantages of this type of optical encoder.

The exemplary embodiments as shown in FIG. 3 a and 3 b provide someexamples of aspects of the invention, however numerous variations arepossible. The motion control system can comprise dedicated hardwareand/or can comprise a suitably programmed programmable device such as amicro controller, micro processor, etc.

The coordinate system can comprise any orthogonal or non orthogonalcoordinate system. As the coordinate system is not “physically present”,the dimensions (i.e. the direction of each axis) of the coordinatesystem can be chosen freely as to meet the requirements set in thisdocument.

In a further embodiment of the invention, the lithographic apparatuscomprises a 2-dimensional encoder in the x-y plane. Due to the largerange of movement in x as well as y direction, gratings of such encoderwould be large, in a practical embodiment approximately equal to a sizeof the surface of the substrate. As it is not desirable that suchgratings interfere with the patterned beam of radiation, the encoder inthis embodiment will be located next to a substrate area of thesubstrate table, hence effectively approximately doubling a size of thesubstrate table.

To further increase accuracy of each of the encoders in any of the aboveembodiments, the encoders may be calibrated. Calibration values (e.g.position correction values or factors) may be stored in a onedimensional (to provide a position calibration along a line) or multidimensional (to provide a calibration in a plane or multi dimensionalspace) calibration matrix. Calibration effectively increases accuracy asabsolute errors tend to be a comparably large source of error inencoders, while other errors, such as thermal stability can be largelysuppressed by other means, such as the use of a low thermal expansionmaterial as described above.

The method for calibrating the position detector in the lithographicapparatus according to the invention will now be described withreference to FIG. 4. In task 100, with a reference lithographicapparatus, a first pattern is created on a substrate, the first patterncomprising a first matrix of reference marks. Then, in task 101, withthe lithographic apparatus comprising the to be calibrated positiondetector, a second pattern is created on the same substrate, the secondpattern comprising a second matrix of reference marks.

In task 102, the second matrix of reference marks is compared with thefirst matrix of reference marks. Should in this step a perfect matchbetween the first and second matrix of reference marks be detected, thenthat step of the procedure would have provided a perfect match, i.e. nocorrection required, however commonly position deviations are determinedbetween the reference marks of the first matrix and the correspondingreference marks of the second matrix, see task 103. The positiondeviations are then stored in the calibration matrix (task 104). Shouldno position deviation have been observed, then a value of zero is storedin the calibration matrix. Commonly, the tasks 102-104 are repeated forindividual reference marks of the first and second matrix of referencemarks, however it is also possible that these steps are performed for aplurality of reference marks in parallel.

With the method according to the invention, it is possible to create ahigh amount of matching between various lithographic apparatuses. Thereference lithographic apparatus may be calibrated in absolute terms,however it is also possible that the reference lithographic apparatushas obtained no specific highly accurate absolute calibration, themethod according to the invention resulting in a high matching betweenthe lithographic apparatus with the to be calibrated position detectorand the reference lithographic apparatus: in lithography, matchingbetween various lithographic apparatuses is mostly of a higherimportance then absolute accuracy of each of the apparatuses as such.

The method according to the invention may also be applied with a singlelithographic apparatus, i.e. the second lithographic apparatus being thefirst lithographic apparatus. The second pattern is in that case createdby rotating the substrate by substantially 90 or 180°, the secondpattern is being created with the same matrix of reference marks on thesame wafer. By a rotation of substantially 90°, a so-called x-to-y ory-to-x calibration can be obtained. By a rotation of substantially 180°,an averaging of position errors can be achieved with the calibrationmethod according to the invention.

Also, when making use of a single lithographic apparatus, i.e. thesecond lithographic apparatus being the first lithographic apparatus, aso-called fishbone technique can be used, the substrate or wafer beingtranslated over a distance of substantially one reference mark, or aplurality thereof, after creating the first pattern and before creatingthe second pattern. The second pattern is in this manner, displaced withrespect to the first pattern by a distance of one reference mark in anideal case.

By comparing an appropriate reference mark of the second matrix with anappropriate reference mark of the first matrix, a position deviationbetween the reference marks of the two matrixes can be obtained. Thisposition deviation is stored in the calibration matrix. With thefishbone technique, in contrast to the embodiments of the calibrationmethod as described above, the pattern is usually small compared to thesurface area of the substrate.

With the embodiments as described earlier in this document, a matrix ofe.g. 100×100 reference marks is applied, while with the fishbonetechnique use is made of a matrix of e.g. 3×3 reference marks of 4×4reference marks. The steps of the fishbone calibration as describedabove, thus only cover a small part of the surface of the substrate.Therefore, further patterns are created with the lithographic apparatuson the substrate, each pattern being translated with respect to theprevious one over a distance of substantially one or more referencemarks.

In this manner, a step by step calibration can be performed, a referencemark of a pattern being compared with a reference mark of a previouspattern, position deviations between these reference marks aredetermined and stored in an appropriate position in the calibrationmatrix. It will be clear to the skilled person that with the fishbonetechnique an overlapping of the reference marks of a pattern with aprevious pattern is required, thus in practice the translation beingover a distance of substantially 1, 2 or 3 reference marks, when amatrix of e.g. 3×3 or 4×4 reference marks is applied.

Further to the variants as described above, it is also possible tocalibrate the position detector of the lithographic apparatus bymounting the position detector in a lithographic apparatus thatcomprises an interferometer position detector for detecting a positionof the substrate table as well as the encoder as described above. Theencoder can now be calibrated with the interferometer. Also, as afurther variant for the calibration, it is possible to use a calibrationmatrix as determined for a position detector with any of the methoddescribed, as a starting value for calibration of a second positiondetector in another lithographic apparatus. This has proven to beadvantageous and provide a good starting point as in practice a matchingbetween position detectors, especially when produced in a sameproduction batch, appears to be high.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention—rather the scope of the invention is defined by the appendedclaims.

1. A lithographic apparatus, comprising: a substrate table configured tohold a substrate; an illuminator configured to condition a beam ofradiation; a support structure configured to support a patterning devicethat imparts a desired pattern to the beam of radiation; a projectionsystem that projects the patterned beam onto a target portion of thesubstrate; and a motion control system configured to control a movementof the substrate table, the motion control system comprising a pluralityof position detectors that detect a position of the substrate table, atleast three of the position detectors each including a single ormulti-dimensional optical encoder to provide at least six positionvalues, the optical encoders being coupled to the substrate table atdifferent locations within a three dimensional coordinate system,wherein at least one position value is provided for each dimension ofthe three dimensional coordinate system, and wherein the motion controlsystem is configured to calculate the position of the substrate tablewithin the three dimensional coordinate system from a subset of at leastthree of the six position values and to calculate an orientation of thesubstrate table with respect to the three dimensional coordinate systemfrom another subset of We at least three of the six position values. 2.The lithographic apparatus of claim 1, wherein the motion control systemcomprises a calibration matrix to calibrate the position detectors. 3.The lithographic apparatus of claim 1, wherein the position detectorscomprise: a first two-dimensional encoder configured to measure aposition of the substrate table in a first and a third dimension of thethree dimensional coordinate system, the first encoder beingmechanically coupled to a first side of the substrate table where thefirst side is substantially parallel to the first dimension, a secondone-dimensional encoder configured to measure a position in the thirddimension of the three dimensional coordinate system, the second encoderbeing mechanically coupled to the first side of the substrate table,wherein the first and second encoder are coupled to opposite parts ofthe first side with respect to a middle of the first side, and whereinthe motion control system determines a position in the first dimensionfrom a position value of the first encoder and determines a rotation ofthe substrate table around the second dimension from position values ofthe first and second encoders.
 4. The lithographic apparatus of claim 3,wherein the position detectors further comprise: a third two-dimensionalencoder configured to measure a position of the substrate table in asecond and the third dimension of the three dimensional coordinatesystem, the third encoder being mechanically coupled to a second side ofthe substrate table where the second side being substantially parallelto the second dimension, a fourth one-dimensional encoder configured tomeasure a position in the third dimension of the three dimensionalcoordinate system, the fourth encoder being mechanically coupled to thesecond side of the substrate table, wherein the third and the fourthencoder are coupled to opposite parts of the second side with respect toa middle of the second side, wherein the motion control systemdetermines a position in the second dimension from a position value ofthe third encoder and determines a rotation of the substrate tablearound the first dimension from position values of the third and fourthencoders.
 5. The lithographic apparatus of claim 3, further comprising:a fifth one-dimensional encoder configured to measure a position of thesubstrate table in the first dimension, the fifth encoder beingmechanically coupled to an end of the third side of the substrate table,the third side being substantially parallel to the first dimension andopposite to the first side of the substrate table, the motion controlsystem determining a rotation of the substrate table around the thirddimension from position values of the first and fifth encoders.
 6. Thelithographic apparatus of claim 1, wherein the optical encoderscomprise: a first two-dimensional optical encoder configured to provideposition values in the first and third dimension of the threedimensional coordinate system, the first optical encoder being coupledto the first side of the substrate table that is substantially parallelto the first dimension; a second two-dimensional optical encoderconfigured to provide position values in the second and third dimensionof the three dimensional coordinate system, the second optical encoderbeing coupled to the second side of the substrate table that issubstantially parallel to the second dimension; a third two-dimensionaloptical encoder coupled to the third side of the substrate table that issubstantially parallel to the first dimension and opposite to the firstdimension.
 7. The lithographic apparatus of claim 6, further comprising:a fourth, one-dimensional optical encoder configured to provide aposition value in the third dimension, the fourth optical encoder beingcoupled to a fourth side of the substrate table, the fourth side beingsubstantially parallel to the second dimension and opposite to thesecond side.
 8. The lithographic apparatus of claim 1, wherein theoptical encoder comprises a diffraction-type encoder comprising: a lightbeam generator constructed for generating a light beam, a first grating,and a second grating, the second grating being movable with respect tothe first grating, and a detector arranged for detecting a diffractedbeam of the light beam as diffracted on the first and the secondgrating, one of the gratings being mechanically connected to thesubstrate table, the other one of the gratings being mechanicallycoupled to a reference base of the lithographic apparatus, wherein amovement of the substrate table causes a movement of the first gratingwith respect to the second grating and in operation causing a change inthe diffracted beam.
 9. The lithographic apparatus of claim 8, whereinthe gratings comprise a plate made of a low thermal expansion material.10. The lithographic apparatus of claim 8, wherein the gratings comprisea channel coupled to a fluid circulation system comprising a thermalstabilization unit that stabilizes a temperature of the grating byacting on a temperature of the fluid in the fluid circulation system.11. The lithographic apparatus of claim 1, wherein at least one of theposition detectors comprises a two-dimensional optical encodercomprising: a grating mechanically coupled to the substrate table inorder to follow a movement of the substrate table, and two opticaldetectors, each of the optical detectors cooperating with the grating inorder to detect a movement of the grating along a different dimension.12. The lithographic apparatus of claim 11, wherein the gratingcomprises a plate made of a low thermal expansion material.
 13. Thelithographic apparatus of claim 11, wherein the gratings comprise achannel coupled to a fluid circulation system comprising a thermalstabilization unit that stabilizes a temperature of the grating byacting on a temperature of the fluid in the fluid circulation system.14. A method for calibrating a position detector, comprising: creating afirst pattern on a substrate comprising a first matrix of referencemarks; creating a second pattern on the substrate comprising a secondmatrix of reference marks; comparing the second matrix of referencemarks with the first matrix of reference marks; determining respectiveposition deviations between reference marks of the first matrix andcorresponding reference marks of the second matrix; and storing theposition deviations in the calibration matrix.
 15. The method of claim14, wherein the substrate is rotated by substantially 90 or 180 degreesbefore creating the second pattern.
 16. The method of claim 14, whereinthe substrate is translated over a distance of substantially one or morereference marks before creating the second pattern.